Lead enrichment in Neotethyan volcanic rocks from Iran: The implications of a descending slab

Basic-intermediate volcanic rocks from the Karaj-Danesfahan area in the Neotethyan magmatic belt of Iran show typical island arc geochemical signatures. The rocks demonstrate calcalkaline affinity with low abundances of "high field strength elements" (HFSE) such as Nb, Ta, Hf and Ti, and highly distinctive spiked trace element patterns at "large ion lithophile elements" (LILE) such as Ba, K, and Sr. Comparing the geochemical characteristics of two sets of the volcanic rocks sampled from two parts of the study area 100 km apart, however, indicate subtle but significant geochemical differences which bear important petrogenetic implications. One set of the volcanic rocks is abnormally enriched in Pb and shows consistently higher abundances of HFSE, particularly Ti, than the other set. These two series are called HTL series (after high titanium and lead) and LTL series (after low titanium and lead), respectively. HTL series volcanic rocks are further from the subduction axis, so they should have had a deeper descending slab contributing to their mantle wedge magmatism. To be able to release Pb-enriched fluids, the subducting slab should not have undergone earlier dehydration. It is because Ph is highly incompatible. It appears that in the early stage, subduction proceeded at a higher rate (cold slab), so slab-dehydration occurred at greater depths triggering HTL series magmatism. Subsequently the subduction rate decreased which in turn raised isotherms and promoted partial melting at shallower depths leading to LTL series magmatism.